RCU Forums

RCU Forums (https://www.rcuniverse.com/forum/)
-   Giant Scale Aircraft - 3D & Aerobatic (https://www.rcuniverse.com/forum/giant-scale-aircraft-3d-aerobatic-110/)
-   -   GSP Sukhoi very tail heavy. HELP!!! (https://www.rcuniverse.com/forum/giant-scale-aircraft-3d-aerobatic-110/4919981-gsp-sukhoi-very-tail-heavy-help.html)

dirtypool 10-26-2006 11:29 PM

GSP Sukhoi very tail heavy. HELP!!!
In assembling a GiantScalePlanes.com 80" Sukhoi, the CG is supposed to be at approx 4.5" behind the LE. It balances at about 7" back. Approximately 5 pounds of lead is required in the nose for balance.

That is CRAZY.

Servos go in the tail - they are counter sunk in the horiz stab. would be difficult to move. could do a pull pull on the rudder, but that would only gain me a few oz.

Moving the wing back would be a major undertaking.

I have a lightweight 40cc gasser in there right now.

The AUW is supposed to be at 15 pounds. It is currently at 15 pounds. Adding that much weight would suck.

Any suggestions?

Fuse is fiberglass, vert stab is fiberglass and hollow. I would need to remove about a pound of weight from the tail.

These calcs are for 25% MAC.

Anyone else have this plane?

budgetdude 10-27-2006 12:14 AM

RE: GSP Sukhoi very tail heavy. HELP!!!
1 Attachment(s)
A pull pull will gain you more than you think. You could also put a heavier ignition battery on a stick and extend it to the leading edge of the cowl.

majortom-RCU 10-27-2006 12:20 AM

RE: GSP Sukhoi very tail heavy. HELP!!!
My GSP 80" Sukhoi ARF is still in its box, so these comments are conceptual rather than based on actual experience. First off, a 25% MAC balance point is the forward limit. I would be looking to test-fly at about 33% MAC, which would be 5.94" by calculation, so 5-15/16" would be my first set-up. Second, it sounds like the tail servos need to come up under the canopy. That would get 6-8 oz out of the tail. That would mean covering over the servo sockets in the stabs and running dual elevator pushrods from under the canopy, and putting pull/pull cables on the rudder. Also look around for the lightest tailwheel you can find. With servos under the canopy, light tailwheel you could find yourself pretty close to 5-3/4" balance point. If you can get it balanced anywhere in the 5-1/2" to 6" range, I'd give it a test flight and see what changes you might want to make after that. No way do you want to be flying a 20 lb aerobat on 40 cc gasser. It would fly, but nothing like a Sukhoi.

dirtypool 10-27-2006 11:28 AM

RE: GSP Sukhoi very tail heavy. HELP!!!

I'll see what would be required (elevator horns) to move the servos up. Even if I use carbon fiber pushrods for the elevators I will probably only gain a few ounces - may save me a pound up front. Then moving the cg back might make it.

I have done the battery on a stick before with success. I was just so far off on this one that it wasn't enough to help. But with all the suggestions above, I may just make it.

Thanks again.


ifshnee-RCU 10-28-2006 09:02 AM

RE: GSP Sukhoi very tail heavy. HELP!!!
Had same problem with an edge. I made pull-pull on the rudder & the elevators. Wasn't that hard to do. Adding weight to an aerobatic airplane is an oxymoron.

dirtypool 10-29-2006 02:37 AM

RE: GSP Sukhoi very tail heavy. HELP!!!
yeah, not a fan of adding more than a few oz if possible.

May look into moving the wing. Not sure how that would affect the model. You move the balance point back, but you also move the weight of the wing back.

Pulling the servos out of the tail and balancing at 33% MAC, I still need to add more weight plus the servos will still be behind CG which would require additional weight.

Project going on the shelf for now. Need to regroup before making another run on this one.

Considering selling the plane....


majortom-RCU 10-29-2006 03:25 AM

RE: GSP Sukhoi very tail heavy. HELP!!!
Well dp your post is a good heads up for me, so thanks for that. I have a G62 I can put in my GSP Sukhoi, so maybe that with forward tail servos will be enough weight up front to get the balance right. Otherwise moving the wing back would certainly do the trick, but that's a bit of a hassle. Another thought is using up all the length of the cowl, extending the engine further forward with longer stand-offs. A certain amount of challenge is stimulating for the brain, but some of these problems are just a pain in the neck. This may help explain why this is not a popular model.

dirtypool 10-29-2006 01:28 PM

RE: GSP Sukhoi very tail heavy. HELP!!!

"stretching" the nose has served me well in the past. I had to do that on my BME Edge. The gap between the cowl and spinner is a little big, but I can live with it.

I am not sure which would be less work. Either way it is gonna be a pain.

Using a lightweight 45cc motor has me at 15 pounds already. That G62 will put you up there even higher. With all these tricks, it may just work out for you. I actually considered getting that motor for it. If that was all it took I would have done it but even with that I still would have needed to move the servos etc.

I don't mind being a little overweight but at some point it gets so heavy it turns into a warbird.

dirtypool 06-03-2007 11:22 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
Anyone else had better luck with these?

I would like to see it fly.

majortom-RCU 06-04-2007 12:34 AM

RE: GSP Sukhoi very tail heavy. HELP!!!
By an amazing coincidence I had a PM today from a guy in Kansas who has the GSP Sukhoi ready to put in the air, powered with FPE 2.4. He was looking for anyone who had flown this plane, and PM'd me after seeing my comments in this thread. I will alert him to maybe post back here after he has maidened (which sounds like it is imminent), and see what he has to say. He could be making RCU history, as far as I know. I was excited when I saw a photo he e-mailed me of the plane all put together, ready to go fly.

Mine is still in the box, waiting only for its owner to get a kick in the butt and a little inspiration.

hpelsromantic 06-04-2007 08:44 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
1 Attachment(s)
well at the risk of being nervous, to say the least, about putting it in the air. I feel like a test pilot flying something that nobody knows what to expect. I'm the guy in kansas.... (no jokes about Dorothy, ok) The pos manual that comes with the plane says cg should be 145 to 150 mm back from the leading edge. So, what I've done so far to get it closer is gone with a FPE 2.4 engine. I built up behind the firewall with a piece of 3/8 ply and moved the rudder servo to make it a pull/pull. It ballances without any extra lead at approximately 6 1/2" but that's getting ready to change tonight. I'm putting on a new fpe 2.4 with a ch upgrade ignition and I'm going to put the ignition and battery in front of the firewall. Also i'm moving from a 7/8" standoff to 1 3/16 standoff. We'll see where that puts us. Comments or tips are welcome..

dirtypool 06-04-2007 09:12 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
I hope that will be enough to get it there.

Please let us know where it balances after the changes listed above.



majortom-RCU 06-04-2007 09:25 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
I picked up a little tip from a kit/ARF mfr in our area for balancing big planes, which is so simple I slap myself for not having thought of it: balance your plane in your usual manner, with wing on, then remove the wing and balance again with just the fuselage. Note the difference, and then make and check all your adjustments with the wing off--much easier to handle. Another tip is to refer all CG measurements to front or back of the wing tube.

hpelsromantic 06-04-2007 11:59 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
when i put the wing together it didn't have a wing tube. It did have an empty tube that houses the servo extentions. i asked dirtypool about measuring back from the leading edge.... a bit of confusion as to the location of the leading edge. the wing is tapered and has a portion of the center section that is notched out with a light ply rib that is epoxied together that fits inside of the fuse in a cutout. My question is should you measure back from the plane of the leading edge of each wing where they intersect or measure back from the leading edge at the fuselage? Dumb question? the manual onle shows the profile of the wing without the fuselage. If its from the fuse side then I've got it to balance at 5" with the 1 3/16 stand offs. I think I'm going back to the shorter standoffs to note the difference. thanks for all your guys inputs.

majortom-RCU 06-05-2007 02:40 AM

RE: GSP Sukhoi very tail heavy. HELP!!!
I begin to think we may not be talking about the same plane here. The GSP/China Model Products Sukhoi I have is an ARF with molded fiberglass fuselage, two-piece wing and aluminum wingtube, such that the two wing halves are mounted to the fuselage for each outing, then removed for the trip back home. This wing is 80" span as mounted to the plane. It sounds like you are talking about a one-piece wing, probably smaller than 80" span, two halves glued together as part of initial assembly, then mounted into a big slot in the fuselage for flying, and removed in one piece for the trip home.

In any case, CG specifications are usually given as distance back from the leading edge as measured with the wing attached to the fuselage, and from the intersection of the wing LE with the side of the fuselage--not from the front of the notch that nestles between the sides of the fuselage.

The best way to assure yourself of having the right measurement is to plot your own Mean Aerodynamic Chord. There are several webpages devoted to this procedure, each with their own way of putting the basic concept. One good one is at [link]http://www.palosrc.com/instructors/cg.htm[/link]. Another is at [link]http://www.nsrca.org/technical/tip_tricks/mac_cg.htm[/link] . The general practice for aerobatic models is to start with CG at 33% of MAC. Some say 30%, some say 35%. There is no 'right' answer, since we are only looking for a close approximation to do our maiden and trim flights at, after which we will move things forward or aft or add ballast to get the plane trimmed to our personal taste and flying style.

After you've calculated or plotted the 33% point, you can verify that you are on target by noting that this point will be very close to either the wingtube if you have a two-piece wing, or to the main spar of a one-piece wing. If you can't see the main spar directly, it will be at the thickest part of the wing as measured vertically. It will also be just forward of the point where the balsa sheeting ends and the film-covered rib bays begin. After somewhere over a hundred kits & ARFs, I've yet to see any model that did not conform to this placement of the main wing spar.

dirtypool 06-05-2007 11:44 AM

RE: GSP Sukhoi very tail heavy. HELP!!!
Good call majortom.

My light didn't even go on when he said that. The pic even shows what appears to be a bolt on wing, not a tube.

Maybe they got a new version.

watcher 101 06-05-2007 12:44 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
It is such a shame that a manufacture is selling these poorly designed airplanes, any ARF that is that far out of the CG envelope is just not worth having, and I really do feel for you guys that have been stung by this manufacturer.:(:(:(


dirtypool 06-05-2007 12:55 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
Thanks Dave.

It is a bit ridiculous that there is no response from GiantScalePlanes.com on this. I can deal with poor service but to sell something that won't even fly is fraudulent. It is like buying a camcorder and receiving a box with nothing but a brick in it.

I guess a credit card dispute was in order.

hpelsromantic 06-05-2007 07:58 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
my apologies to all. You would think after 42 years i could read for myself. I do have a gsp/chinamodels sukhoi but it is of the 69 1/2 inch variety. I just saw the 80" on the post and well...uh.... you know the rest. I searched the forums and this plane is seldom mentioned. the fuse is fiberglass though and it does look nice. I supposed I maybe did a little typing before thinking. Sorry to get you guys excited. nonetheless I still have had the same issues as the larger planes with ballance as it recomends my plane to be a 120 to 140 4c engine. I know it's way overkill but heck, that's why the throttle can be pulled back. Sorry Norm and dirtypool.

dirtypool 06-05-2007 09:40 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
No apologies. Heck, I wish my mistakes were only that small. :)

That motor will certainly make that thing fun to fly. Just watch the wing loading, may need to land a bit hot.

At least the thread got a good bump!

Please let us know how your maiden goes. At least we know one of their Suk's can fly!

hpelsromantic 06-05-2007 10:19 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
dirtypool. I don't know if this might help your confidence but this thread might have some interesting info for ya'. at least there's a couple of people that say they like their planes from gsp

majortom-RCU 06-05-2007 10:25 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
Waall I'm sitting here figuring the 69.5" wing has maybe 900 squares in it, and Brent thinks his plane will weigh no more than 12 lbs, which would produce a wing loading of something like 31 oz/sq ft. That's the same loading I flew my Creek Sukhoi at, which wasn't bad at all. I enjoyed that plane for two seasons, did a lot of good flying with a YS 1.20, APC 17x6. Just not quite enough power for all I wanted it to do, so I retired it to use the engine on another project. So yes, probably a bit hot on the landing, but could be a very workable combination with the FPE 2.4. I guess my only suggestion now would be to balance the prop well and tune the engine for least vibration, since there's not a lot of mass in the airframe to absorb gasser type vibration.

hpelsromantic 06-05-2007 10:54 PM

RE: GSP Sukhoi very tail heavy. HELP!!!
Well, i gotta be honest with ya, the first thing i did was throw away the hardware that came with the arf. i used all heavy duty aluminum control horns and beefed up the firewall with 3/16 lite ply along with it being pinned with wood dowls. I put a little bigger tires and wheels and a new tailwheel assembly. I built a box for the p/p rudder servo and moved the throttle servo back along next to it just aft of the te of the wing. I beefed up the backing for the wing bolts and also at the landing gear. Just installed the new motor tonight and mounted the ignition and it's battery otside the firewall. The new motor just returned from fpe after being gone through and checked out so I'm pretty sure there wont be any problems with it. The new motor has a bosch cap and plug wire so i hope that is where the rf glitch I had with the old motor came from. The weather is supposed to be crummy and windy tomorrow but Saturday might be its maiden at our club picnic. That way i can have even more pressure on me with everyone there to watch..... We'll see how she'll do. I'm going to another club tomorrow to try and get a good range check. Oh, I did put a plastic throttle cable on from the start too. Special thanks is in order to Norm for being so thorough in his help with my pre flight. Dirtypool, thank you as well fo taking a newbie under your wing and helping out too.

All times are GMT -8. The time now is 09:45 PM.

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.